Exhaust gases produced by burning fuels using air as the source of oxidant typically contain small but significant amounts of various nitrogen oxides (NO, NO.sub.2, N.sub.2 O.sub.3, etc.) collectively and interchangeably known as NO.sub.x. NO.sub.x is a participant in the photochemical reaction creating modern "smog" and, therefore, is undesirable.
There are a number of ways in which the NO.sub.2 may be removed, treated, or even its initial synthesis prevented; however, each such process strongly benefits from use of an accurate monitor for detecting low levels of NO.sub.x. The detector may be used in a variety of processes, e.g., closed-loop procedures controlling one or more reaction parameters (such as catalyst temperature, NO.sub.x reductant concentration, or exhaust gas recirculation) all based on NO.sub.x concentration.
There are, however, few low level NO.sub.x sensors available which are practically suitable for inclusion in closed-loop controllers. A major problem of the prior measurement devices has been the lack of sensitivity. An ability to measure NO.sub.x content in combustion gases below 150 ppm with some accuracy is desirable. The major currently available on-line NO.sub.x process analyzers capable of measuring low levels of NO.sub.x are the chemiluminescent analyzers. Such analyzers are expensive, require extensive pretreatment of the gas to be analyzed, and historically have proven to be high maintenance devices. The accuracy of many NO.sub.x sensors, including chemiluminescent sensors, suffer because of interference from other gases which may be found in a combustion gas stream, e.g., SO.sub.2, CO, H.sub.2, H.sub.2 O, and various hydrocarbons.
There are a number of methods known for detecting NO.sub.x in flowing gas streams. Perhaps the most well-known of the processes currently used involve the chemiluminescent reaction of nitric oxide and ozone. Ozone is highly reactive and often raises reactivity problems with analyzer components, e.g., O-rings, metals used in the analyzer, and the like. The process operates by the reaction of injected ozone and the nitric oxide in a sample within a reaction chamber having a transmissive window through which the light produced by the chemiluminescent reaction is monitored by a detector. The window in the reaction chamber must be kept clean to maintain analyzer sensitivity and calibration. Water causes substantial problems in these devices. Typical apparatus used in this process may be found in U.S. Pat. Nos. 3,967,933 to Etess et al.; 4,236,895 to Stahl; 4,257,777 to Dymond; 4,315,753 to Bruckenstein et al.; and 4,822,564 to Howard. The use of a chemiluminescent nitrogen oxide monitor device in controlling a nitrogen oxide removal unit the outlet of a boiler is shown in U.S. Pat. No. 4,188,190 to Muraki et al. The devices and processes disclosed herein could be substituted for the nitrogen oxide analyzers shown in Muraki et al.
Another procedure involves the use of an infrared beam, detector, and a comparator chamber. In U.S. Pat. No. 4,647,777 to Meyer a beam of infrared light is passed through a gas sample and into a selective infrared detector. The beam is split and one portion passes through a chamber containing a fluid which absorbs the spectral wavelengths of the selected gas. The two beams are compared and the difference between the two beams gives an indication of the amount of selected gas in the sample. Although such instruments can measure NO and NO.sub.2, they suffer from the same shortcomings as do the chemiluminescent analyzers in that they require clean optical surfaces, significant sample pretreatment, and have high maintenance requirements.
U.S. Pat. No. 4,836,012 to Doty et al. shows a semiconductor device made up of a photovoltage cell which, upon exposure to light, develops a voltage or current which varies as a function of the type of gas sorbed. The device requires a "thin light-transmitting gas-absorbing metal Schottkey layer having electrical properties which vary with the type of gas absorbed". Detection of CO, hydrocarbon, water vapor, etc. is suggested; detection of NO is not.
Other methods of determining the trace elements of NO.sub.x which may be present in a gas stream are known. For instance, U.S. Pat. No. 3,540,851 to Vree et al. suggests a process in which a gaseous mixture containing substituents such as carbon oxides, nitrogen oxides, sulfur oxides, and oxygen is separated into two streams. One stream is desirably mixed with a ballast gas and sent into a reference arm of a measuring apparatus; a second stream is passed after mixing both with nitrogen and a carrier gas, such as helium, and subjected to an electric discharge. The thus treated gases are passed through a conventional electrometer. The excited NO.sub.x passes to an ionic state and gives off a measurable electron.
U.S Pat. No. 4,115,067 to Lyshkow suggests a process for using a substrate which is sensitive to the pollutant to be measured and monitoring the change in color or reflectivity of the sensitized substrate. Lyshkow suggests the use of a substrate upon which silica which has been impregnated with a mixture of sulfanilic acid and N-(1-naphthyl)-ethylenediamine dihydrochloride. The mixture reacts with NO.sub.2, changes the color of the substrate, and decreases the reflectivity of the substrate having the silica gel coating. Lyshkow suggests that the treated substrate be contacted with the gas to be measured and moved at a constant rate past a device which measures the change of reflectivity of the surface. In this way the amount of NO.sub.2 is measured.
The U.S. Pat. No. 4,778,764 to Fine describes a device and a process in which a sample is injected with a solvent into a chromatographic column to separate the various materials present in the sample. The output of the column is then burned in the presence of a variety of detectors for one or more of NO.sub.x, SO.sub.2, CO.sub.2, and halogens.
U.S. Pat. No. 4,840,913 to Logothetis et al. suggests a method for sensing nitrogen oxides, particularly in the exhaust flow of an internal combustion engine. The gas is passed through an oxidation catalyst which is formed over an oxide sensor. The oxidation catalyst is intended to oxidize all reducing species (CO, H.sub.2, hydrocarbons, alcohols, etc.) which are carried in the gas to be measured. Nitrogen monoxide is oxidized to NO.sub.2 as well. The oxidized gas passes to an oxide sensor such as a SnO.sub.2 or ZnO.
U.S. Pat. No. 4,473,536 to Carberg et al. suggests a process for controlling a NO.sub.x reduction process using a NO.sub.x sensor.
None of the above disclosures suggest a process or an apparatus in which a catalytic sensor element is used to detect the presence of a gaseous component.
The concept of using the temperature rise of a gas as it passes through a catalyst bed as an indicator of the content of a component of that gaseous mixture has been shown. For instance, in U.S. Pat. No. 2,751,281 to Cohen, a method is taught for measuring low concentrations of gas impurities (such as oxygen) in the range of 0.0001% to 0.001%. A thermocouple is placed such that a cold reference junction is on the upstream side of a bed of catalyst and the hot junction is placed on the downstream side of that bed. As the gas flows across the catalyst, the temperature of the gas rises, is detected, and the content of the incoming gas calculated. U.S. Pat. No. 3,488,155 to Ayers shows a similar process in which the temperature on each side of a hydrogenation catalyst bed is measured during the flow of a gas containing hydrogen. The temperature difference is related to the hydrogen content of the incoming gas stream.
The U.S. Pat. No. 3,537,823 to Ines suggests a process for measuring the quantity of "smog forming hydrocarbons in a gas sample" by measuring the temperature rise in an oxidation catalyst bed. Moreover, a related process is found in U.S. Pat. No. 3,547,587 also to Ines.
U.S. Pat. No. 3,607,084 to Mackey et al. teaches a process for the measurement of a combustible gas content by locating a pair of wires in a small chamber containing a volume of gas with combustibles therein. One wire is coated with a catalytic mixture of a metal oxide and a powdered metal of the platinum group and the other is apparently uncoated. Electrical power supplies heat to both wires. The difference in resistance caused by the change in temperature of the wire coated with the catalytic mixture provides an indicator of the amount of combustibles in that gas chamber.
U.S. Pat. No. 4,170,455 to Henrie also suggests a method for the monitoring of the hydrogen or oxygen content of a gas stream by measuring the temperature upstream and downstream of an oxidation catalyst.
U.S. Pat. No. 4,343,768 to Kimura shows a gas detector formed using semiconductor technology. The detector uses dual heating elements over a channel adapted for gas flow. One of the heating elements is coated with a "catalytic or gas responsive film" which may be platinum or palladium. The increase in the temperature of the catalytic film is detected in terms of the variation in electrical resistance in the content of the gas stream calculated.
Finally, U.S. Pat. No. 4,355,056 to Dalla Betta et al. suggests a differential thermocouple combustible sensor in which one junction of the thermocouple is catalytically coated and the other junction is not. The gas stream contains such gases as carbon monoxide and hydrogen and is said to be "insensitive to contaminants such as SO.sub.2 and NO".
None of these disclosures teaches the necessity for satisfying the dimensionless criteria specified here nor are any of the processes disclosed sensitive to the presence of NO.sub.x, particularly when used with a reductant such as those disclosed here.
This invention is a sensor assembly having a functionally specific configuration of a catalytic element having an integral thermally isolate temperature measuring device, e.g., a thermistor or RTD (Resistance Temperature Device). Another portion of the sensor may be a temperature reference element.
This sensor configuration, particularly in conjunction with the inventive process, permits fast resolution of the NO.sub.x content of the gas passing by the sensor to a high degree of accuracy and is insensitive to interference from other gas components in the measured stream.